Two recently published papers have raised concerns about exposure to high levels >=16 milligauss (mG)] of magnetic fields (MFs) during pregnancy and the risk of miscarriage. Power frequency MFs are ubiquitous and about 75% of pregnant women were exposed to MF levels >=16 mG. Therefore, if the reported association of prenatal exposure to MFs >=16 mG with the risk of miscarriage (relative risk ranging from 1.8 to 5.7) is true, it will have significant public health implications. To further examine this association, we propose to conduct a population-based prospective cohort study among susceptible and non-susceptible pregnant women. We will over-sample susceptible women defined as having a history of multiple miscarriages. As demonstrated in our preliminary study, a MF effect on miscarriage is greater among susceptible women than among the overall pregnant population. Further evaluating the MF-miscarriage association among both susceptible and nonsusceptible women has the advantage of increasing the sensitivity of detecting a MF effect and providing an opportunity to compare the difference in the magnitude of the MF effect. The study will be conducted among members of the Kaiser Permanente Medical Care Program Northern California Region, a group-model integrated healthcare delivery system with 32,000 births each year. We will recruit a total of 1,000 pregnant women: 500 with a history of multiple miscarriages and 500 without such a history. In-person interviews will be conducted to collect information on known risk factors for miscarriage. Participants will be asked to wear a meter for 24 hours to capture their personal MF exposure from all sources and to keep a diary of her activities. Pregnancy outcomes will be ascertained through a combination of searching computerized databases, review of medical charts, and telephone follow-up. The Cox proportional hazard regression will be used to determine (a) whether exposure to MF levels =>16 mG during pregnancy increases the risk of miscarriage, (b) whether there is a dose-response relationship, (c) whether this association is stronger for early miscarriage (< 10 weeks of gestation), and (d) whether this association differs between women with and without a history of multiple miscarriages. The proposed study will also significantly improve MF exposure measurements including an enhanced meter capability, repeat measurements, identifying sources of high MF exposure, and measurements on a typical day. Our experience and expertise gained from the recently completed study of MF-miscarriage association should provide significant assurance for the success of the proposed study. The findings from the proposed study will provide much needed confirmation of the recently reported results and advance our understanding of a MF health effect.